but I'm sure it happens.
The day you ordered your vouchers (12th December 2005) was Amazon's busiest in the whole of the pre-Christmas trading period. On that day Amazon UK processed orders for a staggering 3.6 million itmes - an average of 41 orders every second. That day the company shipped 256 tons of goods, and a Royal Mail truck left one of its distribution centres every 15 minutes.
The company was delivering up to 480,000 gifts a day in the run-up to the holiday weekend.
None of that helps you with your un-fulfilled order, but it might go some way to illustrating the complexity of the operation, and explain why the occasional glitch occurred.